mass spectrum of 1-chlorobutane fragmentation pattern of ions for analysis and identification of 1-chlorobutane n-butyl chloride image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of 1-chlorobutane

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The mass spectrum of 1-chlorobutane

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of 1-chlorobutane

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Mass spectroscopy - spectra index

mass spectrum of 1-chlorobutane fragmentation pattern of ions for analysis and identification of 1-chlorobutane n-butyl chloride image diagram doc brown's advanced organic chemistry revision notes

1-chlorobutane C₄H₉Cl, ,

For more see Molecular structure, classification and naming of halogenoalkanes (haloalkanes)

Interpreting the mass spectrum of 1-chlorobutane

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 2-chloro-2-methylpropane.

m/z value of [fragment]⁺	65	63	57	56	55	51	49
[molecular fragment]⁺	[CH₂CH₂³⁷Cl]⁺	[CH₂CH₂³⁵Cl]⁺	[CH₃CH₂CH₂CH₂]⁺	[C₄H₈]⁺	[C₄H₇]⁺	[CH₂³⁷Cl]⁺	[CH₂³⁵Cl]⁺

m/z value of [fragment]⁺	43	42	41	39	29	28	27
[molecular fragment]⁺	[CH₃CH₂CH₂]⁺	[C₃H₆]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[CH₃CH₂]⁺	[C₂H₄]⁺	[C₂H₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1-chlorobutane

[M]⁺ is the molecular ion peak (M) with an m/z of 92 corresponding to [C₄H₉Cl]⁺, the original 1-chlorobutane molecule minus an electron, [CH₃CH₂CH₂CH₂³⁵Cl]⁺. Since this ion is so unstable, there is even less chance of observing the m/z 94 M+2 ion [CH₃CH₂CH₂CH₂³⁷Cl]⁺ (see note below on isotopes).

You might just see a tiny peak at m/z 94 (M+2) corresponding to another molecular ion containing the heavier isotope of chlorine, [CH₃CH₂CH₂CH₂³⁷Cl]⁺.

Note the expected 3:1 ratio of intensities expected for chlorine containing fragment ions.

Since chlorine has two common isotopes of ³⁵Cl and ³⁷Cl in the approximate ratio of 3 : 1, you should observe double peaks in the intensity ratio 3 : 1, two mass units apart for molecular fragments containing a chlorine atom from the fragmentation of 1-chlorobutane.

Two examples of this are quoted above for m/z 63 and 65, 49 and 51, and 35 and 37, although small, you can see their intensities are roughly in the ratio 3 : 1.

Note the molecular ion peaks (M and M+2) are very small indicating the parent molecular ion of 1-chlorobutane fragments very easily.

Atomic masses: H = 1; C = 12; Cl = 35 or 37 (3:1)

Bond enthalpies kJ/mol: C-C = 348; C-Cl = 338; C-H = 412

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of 1-chlorobutane

Small peaks at m/z 49 equate to the [CH₂³⁵Cl]⁺ ion and at m/z 63 [CH₂CH₂³⁵Cl]⁺.

Tiny peaks at m/z 51 and m/z 65 corresponds to [CH₂³⁷Cl]⁺ and [CH₂CH₂³⁷Cl]⁺.

[CH₃CH₂CH₂CH₂Cl]⁺ ===> [CH₂³⁵Cl]⁺ or [CH₂³⁷Cl]⁺ + CH₃CH₂CH₂

[CH₃CH₂CH₂CH₂Cl]⁺ ===> [CH₂CH₂³⁵Cl]⁺ or [CH₂CH₂³⁷Cl]⁺ + CH₃CH₂

Formed by C-C bond scission in parent molecular ion, low probability due to strong bond enthalpy, the weaker C-Cl bond is more likely to be broken.

Note the expected 3:1 ratio of intensities expected for chlorine containing fragment ions.

The most abundant ion, the base ion peak, m/z 56, is formed by the elimination of hydrogen chloride from the parent molecular ion and equates to an ionised butene molecule.

[CH₃CH₂CH₂CH₂Cl]⁺ ===> [C₄H₈]⁺ + HCl

Note the m/z peak of 57 could correspond with the ion [¹³C¹²C₃H₈]⁺.

Mass changes: 94 - 38 = 56 or 92 - 36 = 56.

m/z values of 43 and 29 correspond to alkyl fragments of the original linear chain formed by scission of the C-C bond in the carbon chain of the 1-chlorobutane parent molecular ion.

[CH₃CH₂CH₂CH₂Cl]⁺ ===> [C₃H₇]⁺ + CH₂Cl

[CH₃CH₂CH₂CH₂Cl]⁺ ===> [C₂H₅]⁺ + CH₂CH₂Cl

One reason why the ionised fragments, not containing chlorine, are more likely to be formed, is the more electronegative chlorine tends to make the chlorine containing fragment retain the electrons.

Formation of m/z 42 ion:

[?]⁺ ===> [C₃H₆]⁺ + ?

Ionised propene molecule formed.

Formation of m/z 41 ion:

[?]⁺ ===> [C₃H₅]⁺ + ?

Formation of m/z 39 ion:

[C₃H₅]⁺ ===> [C₃H₃]⁺ + H₂

Formation of m/z 29 ion:

[?]⁺ ===> [C₂H₅]⁺ + ?

Formation of m/z 28 ion:

[C₂H₅]⁺ ===> [C₂H₄]⁺ + H

Ionised ethene molecule formed.

Formation of m/z 27 ion:

[C₂H₅]⁺ ===> [C₂H₃]⁺ + H₂

Formation of m/z 15 ion:

[CH₃CH₂CH₂CH₂Cl]⁺ ===> [CH₃]⁺ + CH₂CH₂CH₂Cl

C-C bond scission of the parent molecular ion (or other fragment) to free a positively charged methyl group

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C₄H₉Cl NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane image sizes. These four molecules are structural isomers of molecular formula C₄H₉Cl and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, chloroalkanes, alkyl chlorides).


INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm^-1, there are no other great striking differences, but each could be identified from its infrared spectrum. The infrared spectrum of 2-chloro-2-methylpropane is noticeably simpler in the fingerprint region, perhaps due to the greater symmetry of the molecule.


MASS SPECTRA (above): Theoretically, all four can give the parent molecular ions of m/z 92 and 94, but they are all relatively tiny peaks. 2-chlorobutane and 2-chloro-2-methylpropane give a base ion peak of m/z 57. The base ion peak for 1-chlorobutane is m/z 56 and that of 1-chloro-2-methylpropane is m/z 43. Each gives different patterns of pairs of m/z values two mass units apart, in the peak height ratio of 3:1, if the positive fragment contains a chlorine atom (³⁵Cl or ³⁷Cl) e.g look for m/z pairs 49/51, 63/65 and 77/79 in their mass spectra.


1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-chlorobutane four peaks of ratio 3:2:2:2; 2-chlorobutane four peaks of ratio 3:3:2:1, 1-chloro-2-methylpropane three peaks of ratio 6:2:1 and 2-chloro-2-methylpropane gives just one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their ¹H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.


13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-chlorobutane and 2-chlorobutane show four 13C NMR resonances, 1-chloro-2-methylpropane three 13C NMR resonances and 2-chloro-2-methylpropane only two 13C resonances (3 and 2 chemical environments respectively. Therefore 1-chloro-2-methylpropane and 2-chloro-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-chlorobutane and 2-chlorobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

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Associated links

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 1-chlorobutane (n-butyl chloride)

The H-1 NMR spectrum of 1-chlorobutane (n-butyl chloride)

The C-13 NMR spectrum of 1-chlorobutane (n-butyl chloride)

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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